Process for producing high-density and high-strength carbon artifacts showing a fine mosaic texture of optical anisotropy

ABSTRACT

Self-adhesive carbonaceous grains for use in the manufacture of high-density and high-strength carbon artifacts containing 0.5-1.5 wt % of a quinoline-soluble but pyridine-insoluble component and at least 97 wt % of a quinoline-insoluble component and which are prepared by heat-treating in a nonoxidizing atmosphere a mesophase pitch that is obtained by polymerizing condensed polycyclic hydrocarbons or substances containing them in the presence of a superacid consisting of hydrogen fluoride and boron trifluoride. The carbonaceous grains are molded and the mold is baked at a sufficient temperature to achieve its carbonization, with the heating rate being not more than 20° C./h in the temperature range from 400 ° to 600° C. In this way, high-density and high-strength carbon artifacts showing a homogeneous fine mosaic texture of optical anisotropy can be efficiently manufactured in high carbon yield.

This is a division of application Ser. No. 08/350,679 filed Dec. 7, 1994now U.S. Pat. No. 5,484,520.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to self-adhesive carbonaceous grains suitable forthe manufacture of high-density and high-strength carbon artifacts. Theinvention also relates to a process for producing high-density andhigh-strength carbon artifacts showing a fine mosaic texture of opticalanisotropy from self-adhesive carbonaceous grains. More particularly,the invention relates to a process for producing high-density andhigh-strength carbon artifacts characterized by random arrangement ofoptically anisotropic units at the submicron level.

2. Prior Art

Many approaches have heretofore been known in the manufacture ofhigh-density carbon artifacts. They are generally produced by mixingfillers such as pulverized coke, natural graphite and carbon black withbinders such as coal-tar pitch, molding the mixture and baking the mold.The carbonization yield of the conventional binder is very low and thedensity of the mold achieved by single carbonization is accordingly verylow; therefore, the steps of impregnation and carbonization must berepeated until the desired properties are attained. As a furtherproblem, the major light-weight components of the binder will evaporateduring the carbonization to form pores that remain in the mold; suchpores will introduce inhomogeneity in the mold. Additionally, structuredisruption may be caused by the bloating of the binder. To avoid theseadverse effects, the carbonization step must be performed by heating atan extremely slow rate but then this increases the time schedule ofcarbon artifact production to at least 3-4 weeks. The carbonized moldmay be graphitized by further heating at 2500°-3000° C., depending onthe use of the final product. Besides, a period of 2-3 weeks is requiredto perform this step of graphitization. In total, a period as long as2-3 months is taken to manufacture graphitic artifacts prepared fromfillers (e.g. coke) and binders (e.g. coal-tar pitch) through theabove-described complex route.

As the demand for improvements in the performance of specialty carbonartifacts and composites is growing year by year, controlling thestructure and texture of carbon is extremely important for the purposeof enhancing the performance of the final product since thephysicochemical characteristics of the product significantly depend onthe structure and texture of carbon. In the field of specialty carbonartifacts, many R&D efforts have been made to control the shape and sizeof starting grains so that the carbon artifacts will exhibit fine mosaictexture, thereby achieving not only higher density and strength but alsophysical isotropy.

A method is known to attain physical isotropy of the artifact by usingmesocarbon microbeads as a starting material. In this method, theoptically anisotropic small spheres that form in the process of heattreatment of coal-tar, petroleum-derived heavy oils, etc. attemperatures of 350°-500° C. are solvent-extracted from the pitchmatrix, dried, molded under pressure and baked. However, as pointed outin Unexamined Published Japanese Patent Application (KOKAI) No. Hei1-239058, the size of optical unit of the carbon artifact produced bythat method is not smaller than the particle size of mesophase spheres(10-20 μm) and it is impossible to decrease the size of optical unit. Afurther problem with the method is that an extremely large quantity ofextraction solvent is required in the step of separating the spheres. Inaddition, it is difficult to remove the remaining solvent completelyfrom the recovered spherical grains and this can be a cause of crackingor expansion of the mold in the subsequent carbonization step. Further,in addition to the extremely low yield of the spheres obtained bysolvent extraction, it is difficult to control their properties. Inother words, this method is not feasible since it is not easy to preparemesocarbon microbeads of acceptable quality and price on an industrialscale.

Another method is concerned with the grains of a pulverized bulkmesophase of specified properties as a starting material (see ExaminedJapanese Patent Publication (KOKOKU) No. Hei 1-58124). However, thecarbon artifacts produced from the pulverized bulk mesophase show lowbulk density, and no satisfactory performance has been achieved. As afurther problem, the bulk mesophase, which is obtained by coalescing andagglomerating mesocarbon microbeads, has to be separated from the pitchmatrix and many complex processing steps are required to obtain a bulkmesophase of specified properties. It has also been pointed out inUnexamined Published Japanese Patent Application (KOKAI) No. Hei1-239058, supra, that the size of optically anisotropic unit derivedfrom pulverized bulk mesophase in the mold cannot become smaller thanthat achieved by pulverization.

Many cases of the attempt to use particular grains that arepreliminarily modified to exhibit a fine mosaic texture have also beenreported. For example, Examined Japanese Patent Publication (KOKOKU) No.Sho 58-58284 teaches a method in which semi-coke with a mosaic texturecomposed of extremely fine optical units (≦1 μm) is used as a moldingfeed. However, this method involves a complex process includingsolvent-extraction of the feed coal in the presence of hydrogen gas,separation of the extract, followed by heat treatment. Examined JapanesePatent Publication (KOKOKU) No. Hei 3-6448 teaches a method of addingcarbon black to pitch, and Unexamined Published Japanese PatentApplication (KOKAI) No. Hei 1-239058, supra, teaches a process forproducing an isotropic graphite artifact having a homogeneous mosaictexture by the steps of incorporating a resin in pitch, pulverizing themixture, molding the grains in the absence of a binder, and baking themold. However, both methods have the disadvantage of involving a complexprocedure comprising mixing, kneading and re-pulverization. In addition,the carbon artifacts produced by these methods exhibit low bulk density,and no satisfactory performance has been attained.

Extensive studies have also been conducted in the area of carboncomposites. For instance, International Symposium on Carbon, Toyohashi,Extended Abstract, p.196, 1982 reported the superiority of matrix carbonshowing a fine mosaic texture as regards the development of excellentthermal shock resistance and high mechanical strength; CARBON, vol. 28,1990 reports a pitch/phenolic resin system (p.559) and a pitch/carbonblack system (p.143), together with their interaction and carbonizationproperties.

As will be understood from the foregoing discussion, the manufacture ofhigh-density carbon artifacts involves extremely complex andtime-consuming processes and, hence, the products of the conventionalmethods are expensive enough to substantially limit the scope of theirindustrial applicability. Under the circumstances, one major goal incarbon industry in connection with the manufacture of high-densitycarbon artifacts is substantial simplification of the production processwhile shortening the time required for their production. As for thecontrol of carbon texture which is the key factor to the enhancement ofproduct performance and structural homogenization, the conventionalprocesses have had various drawbacks as described hereinabove.

The present inventors previously found that self-adhesive carbonaceousgrains suitable for the manufacture of high-density carbon artifactscould be prepared from a specified mesophase pitch and that this couldbe used as a means to solve the aforementioned problems of the priorart. The inventors formulated their finding in a patent application,which was filed in the United States of America as U.S. Ser. No.08/063,421. This prior application teaches that self-adhesivecarbonaceous grains defined in terms of H/C and O/C values haveoutstanding performance. However, the application makes no reference tothe quinoline-soluble and pyridine-insoluble component or thequinoline-insoluble component of the carbonaceous grains, nor does itteach that carbon artifacts derived from said carbonaceous grains arecharacterized by random orientation of optically anisotropic units atthe submicron level.

SUMMARY OF THE INVENTION

An objective, therefore, of the present invention is to provideself-adhesive carbonaceous grains from which high-density andhigh-strength carbon artifacts can be manufactured at low cost in ashort time.

Another objective of the invention is to provide a process by whichhigh-performance carbon artifacts exhibiting a fine mosaic texture ofoptical anisotropy can be produced from said carbonaceous grains in asimple, economical and consistent way.

With a view to attaining these objectives, the present inventorsconducted intensive studies and found the following: first, thermallymodified pitch grains consisting of a quinoline-soluble butpyridine-insoluble component and a quinoline-insoluble component inamounts of specified ranges which were prepared by the heat treatment ofa mesophase pitch that was obtained by polymerizing a condensedpolycyclic hydrocarbon or a substance containing the same in thepresence of hydrogen fluoride/boron trifluoride could be molded withoutusing a binder, and the grains in their mold maintained its shape in theprocess of carbonization, exhibiting high carbon and fusibility foradhesion yield; secondly, by proper control of the rate of heatelevation at which the molded part of the thermally modified mesophasepitch grains was heated in the early stage of carbonization,high-density and high-strength carbon artifacts exhibiting ahomogeneous, fine mosaic texture could be produced in a short time andin a consistent and economical way. The present invention has beenaccomplished on the basis of these findings.

Therefore, in its first aspect, the present invention relates toself-adhesive carbonaceous grains that contain 0.5-1.5 wt % of aquinoline-soluble but pyridine-insoluble component and at least 97 wt %of a quinoline-insoluble component and which are prepared by the heattreatment in a nonoxidizing atmosphere of a mesophase pitch that isobtained by polymerizing a condensed polycyclic hydrocarbon or asubstance containing the same in the presence of hydrogen fluoride/borontrifluoride.

In its second aspect, the present invention provides a process forproducing a carbon artifact exhibiting a fine mosaic texture by thesteps of molding the above-described self-adhesive carbonaceous grainsunder pressure and baking the mold in a nonoxidizing atmosphere at asufficient temperature to achieve its carbonization or graphitization,with the carbonization rate being not more than 20° C./h in thetemperature range from 400°to 600° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical micrograph of the carbonized product as obtained inExample 2;

FIG. 2 is an optical micrograph of the graphitized product as obtainedin Example 2;

FIG. 3 is an optical micrograph of the carbonized product as obtained inComparative Example 3; and

FIG. 4 is an optical micrograph of the graphitized as obtained inComparative Example 3.

DETAILED DESCRIPTION OF THE INVENTION

The precursor of the self-adhesive carbonaceous grains is a mesophasepitch that is obtained by polymerizing a condensed polycyclichydrocarbon or a substance containing the same in the presence of asuperacid consisting of hydrogen fluoride and boron trifluoride. Theprecursor mesophase pitch is described in Unexamined Published JapanesePatent Application (KOKAI) Nos. Sho 63-146920, Hei 1-139621 and Hei1-254796 and it can be prepared by polymerizing a condensed polycyclichydrocarbon such as naphthalene, methylnaphthalene, anthracene,phenanthrene, acenaphthene, acenaphthylene or pyrene or a substancecontaining the same in the presence of a superacid catalyst consistingof hydrogen fluoride and boron trifluoride.

Hydrogen fluoride, when present in combination with boron trifluoride,forms a strong protonic acid which, in turn, forms a complex with thebasic condensed polycyclic hydrocarbon. Hydrogen fluoride also works asa solvent and the resulting complex dissolves in excess hydrogenfluoride to form a complex solution. The polymerization reaction willproceed very smoothly in this hydrogen fluoride solution under mildconditions. Thus, hydrogen fluoride used in an excess amount functionsprimarily as a catalyst by equally important is its function as areaction medium.

In the preparation of mesophase pitch using the superacid polymerizationcatalyst which consists of hydrogen fluoride and boron trifluoride, theproperties of the mesophase pitch to be prepared can be controlled byproper selection of the polymerization conditions including the reactiontime and temperature, the molar ratio between the condensed polycyclichydrocarbon feed, hydrogen fluoride and boron trifluoride, and the typeof condensed polycyclic hydrocarbon to be used. Normally, naphthalene isused as the starting monomer and subjected to the polymerizationreaction at 200°-300° C. for several hours. Since the catalystconsisting of hydrogen fluoride and boron trifluoride has a very lowboiling point, it can be completely separated from the resulting pitchand, hence, the mesophase pitch obtained exhibit an extremely highchemical purity.

The mesophase pitch under discussion is obtained by cationicpolymerization in the substantial absence of the occurrence ofdehydrogenation and, hence, its structure is characterized by highcontent of naphthenic hydrogen and aliphatic hydrogen (see TANSO, No.155, p.370, 1992). In other words, the mesophase pitch to be used as aprecursor in the present invention for producing self-adhesivecarbonaceous grains is clearly distinguishable in structure not onlyfrom the conventional coal- or petroleum-derived mesophase pitches whichare prepared by thermal poly-condensation of coal tar and petroleumresidues which are by-products in the coal or petrochemical processesbut also from the bulk mesophase that is described in Examined JapanesePatent Publication (KOKOKU) No. Hei 1-58124, supra.

The self-adhesive carbonaceous grains according to the first aspect ofthe invention are produced by heat-treating the above-describedmesophase pitch in a nonoxidizing atmosphere such as a nitrogenatmosphere. The conditions of the heat treatment are not limited in anyparticular way but the mesophase pitch is generally heat-treated attemperatures in the range from 420°to 590° C. It is essential for thepurposes of the present invention that the conditions of the heattreatment be so selected as to form 0.5-1.5 wt %, preferably 0.7-1.3 wt%, of a quinoline-soluble but pyridine-insoluble component and at least97 wt %, preferably at least 98 wt %, of a quinoline-insolublecomponent.

For quantitation of the respective components, pyridine-dependentfractionation is performed by Soxhlet extraction whereasquinoline-dependent fractionation is performed by centrifugation inaccordance with JIS K2425.

In the first aspect of the invention, it is by thermally modifying themesophase pitch of interest so as to contain 0.5-1.5 wt % of aquinoline-soluble but pyridine-insoluble component and at least 97 wt %of a quinoline-insoluble component that excellent molding property isassured for the carbonaceous grains while high-density and high-strengthcarbon artifacts can be manufactured without inducing cracking orbloating in the step of carbonization.

Stated more specifically, if the quinoline-soluble andpyridine-insoluble component which corresponds to the binder componentin the thermally modified pitch and the quinoline-insoluble componentwhich guarantees high carbonization yields are adjusted to lie withinthe above-mentioned ranges by controlled heat treatment, the followingcharacteristics will develop in the carbonaceous grains, which make themsuitable as the starting material for the manufacture of carbonartifacts. First, they will deform appropriately at room temperatureunder pressure and, hence, they can be packed tightly, exhibitingexcellent molding property even at room temperature. The mold of thecarbonaceous grains has a sufficient strength to reasonably withstandordinary handling even if it is yet to be baked. Furthermore, in theearly stage of carbonization, the carbonaceous grains in their moldmaintain its shape and yet they show an appropriate degree of fusibilityfor adhesion; therefore, the individual grains in the mold will adhereto each other very strongly to provide a fine mosaic texture, therebyachieving high density and strength. In addition, the carbonaceousgrains under discussion contain at least 97 wt % of thequinoline-insoluble component, so the yield of their carbonization isextremely high and there will be little formation of pores due to theevolution of volatile matters in the process of carbonization.Consequently, as will be demonstrated later in the Examples of theinvention, the mold thus prepared shows high density and strengthtogether with homogeneous and dense structure.

If the content of the quinoline-soluble and pyridine-insoluble componentwhich corresponds to the binder component of the thermally modifiedpitch exceeds 1.5 wt %, the carbonaceous grains in the mold exhibitexcessive fusibility in the early stage of carbonization, forming a flowtexture where many shrinkage cracks are induced in the later stage ofcarbonization. The further increase in the content of thequinoline-soluble and pyridine-insoluble component may cause deformationor expansion of the mold. It should also be noted that the content ofthe quinoline-soluble and pyridine-insoluble component is complementaryto the content of the quinoline-insoluble component and that, therefore,if the former exceeds 1.5 wt %, the latter becomes lower than 97 wt %,resulting in the decrease in carbonization yield. Consequently, morevolatile matters are prone to evolve, thereby increasing the number ofpores that form in the carbonized mold. Thus, carbonaceous grainscontaining more than 1.5 wt % of the quinoline-soluble butpyridine-insoluble component will produce an artifact with so manycracks and pores, which shows low bulk density and mechanical strength(see Comparative Example 1 which follows).

If the content of the quinoline-soluble and pyridine-insoluble componentis less than 0.5 wt %, the carbonaceous grains in their mold willexhibit insufficiently fusibility in the early stage of carbonization,decreasing the force of adhesion among grains. Consequently, many voidswill form when the individual grains shrink in the later stage ofcarbonization and, what is more, cracks are prone to occur within theregions where optical unit is comparatively large. The resultant moldwill show inhomogeneous texture in which there are many voids andcracks, and its bulk density and mechanical strength (see ComparativeExample 2 that follows) will decrease.

If the mesophase pitch of interest is thermally modified in such a waythat the contents of the quinoline-soluble but pyridine-insolublecomponent and the quinoline-insoluble component are within the statedranges, the resultant grains exhibit outstanding self-adhesive property,molding property, and higher carbonization yield. Therefore,high-density and high-strength carbon artifacts showing a homogeneousand dense texture can be efficiently manufactured even if bakingtreatment is carried out only once.

In accordance with the second aspect of the invention, there is provideda process for producing a high-density and high-strength carbon artifactfrom the carbonaceous grains described hereinabove. The process startswith pulverizing the mesophase pitch that has been heat-treated tosatisfy the requirements also described above. The method of pulverizingthe mesophase pitch and the shape of the grains thus produced are notlimited in any particular way. The grain size distribution also is notlimited to any particular value but it is preferably such that thegrains can be molded at the highest possible packing density. Formolding purposes, carbonaceous grains of 1-200 μm are typically used,with those of 1-20 μm being particularly preferred.

In the next step, the grains of thermally modified mesophase pitch aremolded under pressure, preferably by isostatic pressing. There is noparticular need to use a binder in the molding step. The shape of themold is not limited in any way and can be freely selected depending uponsuch factors as the object and use of the final product. Molding may beperformed either at room temperature or in such a temperature range thatthe thermally modified grains will soften or melt. The selection of asuitable temperature depends on the required shape, performance and costof the final product.

The mold is subsequently baked to produce the desired carbon artifact.The baking temperature for carbonization is generally in the range from600° to 1700° C. The carbonized mold thus obtained may be graphitized at1700°-3000° C. if required. In other words, the carbon artifacts to beproduced in accordance with the second aspect of the invention includetwo types, one being produced by carbonization at 600°-1700° C. and theother being produced by graphitization at 1700°-3000° C.

In order to obtain the carbon artifacts exhibiting a fine mosaic textureafter baking, it is extremely important that the green mold consistingof the self-adhesive carbonaceous grains according to the first aspectof the invention be baked in such a way that the rate of temperatureelevation in the early stage of carbonization from 400° up to 600° C. isno more than 20° C./h, preferably between 5° and 15° C./h.

By adopting such a heating profile, the domain texture of the unbakedgrains can be modified, upon baking, to a homogeneous and fine mosaictexture in which optically anisotropic units are randomly oriented atthe submicron level. The thus formed texture is very small in the sizeof the optically anisotropic unit. Hence, the development andpropagation of cracks in the process of graphitization are effectivelyinhibited. Thereby producing carbon artifacts that are further advancedin performance and which are rendered isotropic in physical properties.

If the rate of temperature elevation in the early stage of thecarbonization (400°-600° C.) of the green mold is faster than 20° C./h,no fine mosaic structure will form and the greater part of the mold willshow a coarse and inhomogeneous texture composed of opticallyanisotropic units whose size is much the same to that of the unbakedgrains in their mold. If further heating is done to achievegraphitization, many cracks will be induced and the desired opticaltexture cannot be obtained.

SPECIFIC EXAMPLES OF THE INVENTION

The following examples are provided for the purpose of furtherillustrating the present invention but are in no way to be taken aslimiting. The carbon artifacts prepared in Example 2 and ComparativeExample 3 are shown in optical micrographs in FIGS. 1-4, for which themagnification was fixed at X400.

Example 1

Naphthalene (7.0 mol), hydrogen fluoride (3.7 mol) and boron trifluoride(1.05 mol) were charged into a 3-L acid-resistant autoclave andsubjected to reaction at 265° C. for 4 h with the pressure kept at 27kgf/cm². The release valve on the autoclave was then opened to recoversubstantially all quantities of hydrogen fluoride and boron trifluoridein the gaseous state under atmospheric pressure. Thereafter, nitrogenwas blown into the autoclave to remove the low-boiling components,producing a mesophase pitch in a yield of 75 wt % (relative to thenaphthalene feed). The pitch was wholly composed of an opticallyanisotropic phase; it had a softening point of 240° C. with a H/C atomicratio of 0.65.

The temperature of the thus synthesized mesophase pitch was raised up to480° C. in a nitrogen atmosphere at a rate of 300° C./h and a heattreatment was subsequently conducted at that temperature for 1 h toproduce a homogeneous, thermally modified pitch. The pitch contained 1.3wt % of a quinoline-soluble but pyridine-insoluble component and 98.1 wt% of a quinoline-insoluble component. The pitch was pulverized with aball mill to grains having an average size of 6 μm; the grains weremolded into a plate (35 mm×40 mm×10 mm) under pressure of 1.5 tf/cm² atroom temperature. The plate was heated in a flow of argon with thetemperature being raised from 400° C. to 600° C. at a rate of 20° C./h;after holding at 600° C. for 1 h, the temperature was raised to 1200°C., at which the plate was held for 2 h. The resulting carbonizationyield was 93.2%. The carbonized product was further graphitized at 2000°C. for 2 h. The carbonized product and the graphitized product showedthe physical properties shown in Table 1 below.

Comparative Example 1

A mesophase pitch of the same characteristics as in Example 1 washeat-treated at 465° C. for 1 h to produce a homogeneous, thermallymodified pitch. The pitch contained 1.8 wt % of a quinoline-soluble butpyridine-insoluble component and 96.4 wt % of a quinoline-insolublecomponent. The mold of the pitch grains was then baked under the sameconditions as in Example 1. Because of the excessive content of thequinoline-soluble and pyridine-insoluble component in the thermallymodified pitch, the resultant artifact did not exhibit any highperformance as demonstrated in Table 1.

Comparative Example 2

A mesophase pitch of the same characteristics as in Example 1 washeat-treated at 500° C. for 1 h to produce a homogeneous, thermallymodified pitch. The pitch contained 0.1 wt % of a quinoline-soluble butpyridine-insoluble component and 99.8 wt % of a quinoline-insolublecomponent. The mold of the pitch grains was baked by the same procedureas in Example 1. Because of the insufficiency of the quinoline-solublebut pyridine-insoluble component in the thermally modified pitch, theresultant artifact did not show any high performance as demonstrated inTable 1.

                  TABLE 1                                                         ______________________________________                                        Calcination   Bulk      Compressive                                                                              Flexural                                   temperature,  density,  strength,  strength,                                  °C.    g/cm.sup.3                                                                              kgf/mm.sup.2                                                                             kgf/mm.sup.2                               ______________________________________                                        Ex. 1  1200       1.88      29.5     14.3                                            2000       2.05      24.5     12.8                                     Comp.  1200       1.80      13.4     5.7                                      Ex. 1  2000       1.90       8.2     3.3                                      Comp.  1200       1.70      12.0     5.2                                      Ex. 2  2000       1.77       8.0     3.2                                      ______________________________________                                    

Example 2

Naphthalene (7.0 ml), hydrogen fluoride (2.4 mol) and boron trifluorine(0.74 mol) were charged into a 3-L acid-resisting autoclave andsubjected to reaction at 290° C. for 4 h with the pressure kept at 25kg/cm². The release valve on the autoclave was then opened to recoversubstantially all quantities of hydrogen fluoride and boron trifluoridein the gaseous state under atmospheric pressure. Thereafter, nitrogenwas blown into the autoclave to remove the low-boiling components,producing a mesophase pitch in a yield of 70 wt % (relative to thenaphthalene feed). The pitch had a softening point of 250° C., with anoptically anisotropic phase content of 100% and a H/C atomic ratio of0.60; the carbonization yield was 87 wt %.

The thus synthesized mesophase pitch was heat-treated at 480° C. for 1 hin a nitrogen atmosphere to produce a homogeneous, thermally modifiedpitch. The pitch contained 1.0 wt % of a quinoline-soluble butpyridine-insoluble component and 98.5 wt % of a quinoline-insolublecomponent. The pitch was pulverized to grains having an average size of7 μm; the grains were molded into a plate (35 mm×40 mm×10 mm) underpressure of 1.5 tf/cm² at room temperature. The green plate was heatedin a flow of nitrogen, with the temperature being raised from roomtemperature to 400° C. at a rate of 300° C./h and from 400° C. to 600°C. at a rate of 12° C./h; thereafter, the plate was held at 600° C. for2 h.

The carbonized product thus obtained was mounted in a resin and polishedfor examination under an optical microscope. As shown in FIG. 1, thecarbonized product exhibited a homogeneous and fine mosaic texturecharacterized by random orientation of anisotropic units at thesubmicron level. The carbonized product showed a bulk density of 1.35g/cm³, a compressive strength of 15.6 kgf/mm², and a flexural strengthof 7.8 kgf/mm².

The carbonized product was further heated in a flow of argon, with thetemperature raised up to 1900° C. at a rate of 300° C./h, and held at1900° C. for 2 h to be graphitized. As shown in FIG. 2, the opticaltexture of the graphitized product was even more homogeneous than thatof the carbonized product, and the size of optical units became muchsmaller. No crack was observable in the graphitized product. Itexhibited a bulk density of 2.04 g/cm³, a compressive strength of 25.7kgf/mm², and a flexural strength of 13.7 kgf/mm².

Comparative Example 3

Thermally modified mesophase pitch of the same characteristics as inExample 2 were molded into a plate under the same conditions as inExample 2. The green plate was heated from room temperature up to 600°C. at a rate of 300° C./h and held at 600° C. for 2 h. The thuscarbonized plate exhibited the inhomogeneous texture as shown in FIG. 3;obviously, the size of optical units was large, retaining the size ofas-pulverized grains in many regions. The carbonized product showed abulk density of 1.32 g/cm³, a compressive strength of 12.9 kgf/mm² and aflexural strength of 6.0 kgf/mm².

The carbonized plate was further heated to 1900° C. in a flow of argon,with the temperature raised at a rate of 300° C./h, and held at 1900° C.for 2 h to be graphitized. As shown in FIG. 4, the optical texture ofthe graphitized product was inhomogeneous. Cracks were detectable insome particular regions where optical unit was relatively large. Thegraphitized product showed a bulk density of 1.97 g/cm³, a compressivestrength of 14.3 kgf/mm², and a flexural strength of 5.9 kgf/mm².

ADVANTAGES OF THE INVENTION

The carbonaceous grains of the present invention has many desirableproperties such as excellent deformability under pressure at roomtemperature, shape stability after molding, appropriate fusibility foradhesion in the early stage of carbonization, good graphitizability, andextremely high carbon yield. Because of these properties, the grainswill exhibit particularly excellent performance when used as a singlestarting material for the manufacture of high-density and high-strengthcarbon artifacts. The carbon and graphite artifacts derived from theprecursor mesophase pitch not only exhibit an optically anisotropictexture but also possess a dense, homogeneous and chemically purestructure, thus forming very strong carbon bonds. The strength of suchcarbon bonds is further increased by baking at higher temperature; sincethe degree of graphitization is increased and densification is furtherenhanced by shrinkage during baking. The carbonaceous grains of theinvention which are prepared by thermally modifying the mesophase pitchto an appropriate extend exhibit self-adhesive property and need nobinder. Therefore, carbon artifacts having a homogeneous and fine mosaictexture of optical anisotropy can be efficiently produced from thosecarbonaceous grains by controlling the heating rate at which temperatureis raised in the early stage of carbonization of the mold. Since thiscontributes to the achievement of isotropic mechanical properties andimprovement in performance, high-density and high-strength carbonartifacts can be manufactured in a short time in an efficient andeconomical way.

What is claimed is:
 1. A process for producing a carbon artifact showinga fine mosaic texture of optical anisotropy by the steps of moldingself-adhesive carbonaceous grains into a predetermined shape underpressure and baking the mold in a nonoxidizing atmosphere at asufficient temperature to achieve carbonization or graphitization ofsaid mold, with the heating rate being not more than 20° C./h in thetemperature range from 400° to 600° C., said self-adhesive carbonaceousgrains containing 0.5-1.5 wt % of a quinoline-soluble butpyridine-insoluble component and at least 97 wt % of aquinoline-insoluble component and being prepared by the heat treatmentin a nonoxidizing atmosphere of a mesophase pitch that is obtained bypolymerizing a condensed polycyclic hydrocarbon or a substancecontaining the same in the presence of hydrogen fluoride and borontrifluoride.
 2. A process according to claim 1 wherein the heating rateis 5°-15° C./h.
 3. A process according to claim 1 wherein thenonoxidizing atmosphere for the baking step is argon.
 4. A processaccording to claim 1 wherein the baking temperature is from 600° to3000° C.
 5. A process according to claim 1 wherein the carbonaceousgrains have an average size of 1-200 μm.
 6. A process according to claim5 wherein the average grain size is 1-20 μm.